This invention relates to economical beam-steered array antennas and, more particularly, to such antennas using mechanical beam steering in both azimuth and elevation, and configured to minimize antenna protrusion above the surface of an airframe.
Many types of antennas are available for aircraft and other airborne applications. Particular constraints in choice of antennas for use on commercial airliners, for example, are size, weight, protrusion above the surface and complexity of installation. Cost is also a significant consideration, particularly for applications not pertaining to aircraft operation or safety systems. Such applications include receiving of television programming for airline passenger entertainment.
For applications such as airborne reception of television signals from a satellite, the aircraft antenna must provide an antenna beam pattern which is steerable both in azimuth and in elevation. Beam steering in azimuth and elevation is necessary in order to permit the relatively narrow antenna beam to be pointed at the satellite, so that the antenna can successfully receive a relatively weak satellite signal.
The required capability could be provided by a flush-mounted, electronically steered phased-array antenna providing a fully steerable beam. However, such phased-array antennas are typically both expensive and complex, with respect to the electronic circuitry required. Thus, while electronically beam steered antennas may provide the required operating and low protrusion characteristics, cost is typically a constraint foreclosing use for applications such as entertainment systems. In contrast, mechanical arrangements, such as a rotatable antenna with dish reflector, are less expensive in so far as the basic antenna is concerned. However, in an aircraft installation a rotating dish typically has a large, above-surface protrusion requiring a large radome and resulting in unacceptable drag and other disadvantages which are controlling for aircraft installations. Alternatively, a dish antenna can be internally located below the aircraft skin to reduce radome height, however this requires cutting a large hole in the aircraft fuselage which substantially increases installation costs.
In an effort to provide an economical solution to this problem, U.S. Pat. No. 5,420,598, utilizes an array antenna providing a beam normal to the array face. The antenna is mechanically rotated and tilted to enable reception from satellites at any azimuth and over a range of elevation angles. However, with a beam normal to the array face, the antenna of this patent would have to be tilted from horizontal to a tilt angle of 70 degrees to enable reception from a satellite at 20 degrees elevation. To aim the beam at the satellite the antenna must be tilted to the complementary angle of the satellite elevation (90.degree.-20.degree.=70.degree. array tilt). Thus, while objectives are partially met, low antenna protrusion (i.e., low array tilt) for reception from low elevation satellites is not possible with the antennas of this patent.
Objects of the present invention are, therefore, to provide new and improved types of mechanically beam-steered aircraft array antenna systems and such antenna systems providing one or more of the following capabilities and advantages:
fixed-tilt antenna beam from flat radiating array; PA1 array tilt limited by provision of fixed-tilt beam; PA1 20 to 70 degree elevation coverage with array tilt limited to 25 degrees; PA1 no requirement for electronic beam steering; PA1 mechanical beam steering by rotating and tilting the radiating array; PA1 economical individual rotate motor and tilt motor configuration; PA1 omnidirectional azimuth beam steering; PA1 light weight, economical construction using linear array of fixed-focus slotted waveguide radiators; and PA1 small installation hole in aircraft for motor drive, thereby minimizing installation cost. PA1 (a) providing a fixed-beam array antenna configured to provide an antenna beam at a fixed acute angle (.alpha.) above an antenna aperture plane, the array antenna having no adjustable beam scan capability; and PA1 (b) to receive signals from a satellite at an elevation angle (.beta.) above horizontal, mechanically tilting the array antenna to position its aperture plane at a tilt angle nominally equal to the angle complementary to the satellite elevation angle (90.degree.-.beta.), less the acute angle (.alpha.) thereby limiting the antenna tilt angle (90.degree.-.beta.-.alpha.) required for reception from low-elevation satellites. PA1 (c) mechanically rotating the array antenna to direct the antenna beam in an azimuth direction appropriate to receive signals from the satellite.